Washington DC - Democrats in the US House of Representatives introduced an article of impeachment against outgoing President Donald Trump on Monday, as Republicans blocked a measure calling on Vice president Pence to remove his boss from office.

The single impeachment article charges Trump (74) with incitement of insurrection. It accuses him of stoking violence against the US government after a mob of his supporters, riled up by his claims of election fraud, stormed the US Capitol last week.

Democrats also introduced a resolution urging Pence (61) to invoke the Constitution's 25th Amendment and strip Trump of his powers by declaring him unfit to perform his duties. House Republicans rejected the bill, paving the way for a vote in the full House on Tuesday.

House Speaker Nancy Pelosi (80) said that by blocking the measure, Republicans were "enabling the president's unhinged, unstable and deranged acts of sedition to continue."

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"Their complicity endangers America, erodes our Democracy, and it must end," she said in a statement.

If Pence refuses to act, as is expected, House Democrats have vowed to quickly move ahead with impeachment proceedings.

The House could vote as early as mid-week.

The impeachment article notes that Trump repeated debunked claims that he won the November election in a speech before the deadly assault on Congress, which temporarily halted a joint session to certify President-elect Joe Biden's election victory and forced lawmakers to go into hiding.

The article also cites Trump's pressure on officials in the US state of Georgia to "find" him more votes in an effort to overturn his election defeat.

"In all this, President Trump gravely endangered the security of the United States and its institutions of government," the resolution reads. "He threatened the integrity of the democratic system, interfered with the peaceful transition of power, and imperiled a coequal branch of government."

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Trump, who has only nine days left in his term, could become the first US president to be impeached twice. He was impeached in 2019 on allegations of abusing his position, but the Republican-controlled Senate cleared him of charges last year.

If the Democrat-led House voted to impeach the Republican president again, a trial would then be conducted in the Senate, where a two-thirds majority would be needed to convict him. Few Republican lawmakers have so far publicly voiced their support for the moves.

There will almost certainly be no outcome before January 20.

If convicted, Trump could be barred from running for office again in 2024.

In the aftermath of the attack on the Capitol, Trump eventually promised an orderly transfer of power on January 20, the day of Biden's inauguration.

Acting Capitol Police chief Yogananda Pittman said several police officers have been suspended as investigations into the January 6 riot continue.

US Congressman Tim Ryan, who chairs a subcommittee that oversees the Capitol Police, had earlier told reporters that one officer was suspended for taking a selfie with rioters and another for putting on a Make America Great Again hat and directing people.

Ryan said between 10 and 15 other officers were under investigation.

The nation's capital remains on high alert as authorities brace for more possible unrest. Mayor Muriel Bowser on Monday urged people to stay away from the city on the day of the inauguration, while the National Park Service will suspend Washington Monument tours.

Up to 15,000 additional National Guard troops could be deployed there by the weekend, CNN reported, citing National Guard chief Daniel Hokanson.

Meanwhile, Trump approved an emergency declaration for the US capital that runs from January 11 until January 24 – four days after the inauguration – and authorizes the Department of Homeland Security and Federal Emergency Management Agency (FEMA) to coordinate with local authorities.

Pittman said there will be no public access to the Capitol grounds during the Biden's inauguration and that coordinated plans were in place and law enforcement from the federal, state and local levels were working together to ensure safety.

Biden (78) on Monday said he is "not afraid" to take the oath of office outside.

The incoming president also told a journalist that he hopes the Senate can hold an impeachment trial while at the same time confirming his cabinet nominees and passing a coronavirus stimulus package, according to CNN.